1. Field of the Invention
The present invention relates to semiconductor devices, in particular, a semiconductor device that outputs an oscillation signal.
2. Description of the Background Art
A universal microcontroller product requires not only a conventional oscillation control circuit controlling an external crystal resonator, an external ceramic resonator and the like, but also an oscillation circuit (an on-chip oscillator circuit) incorporated in a semiconductor device without requiring an external crystal resonator or an external ceramic resonator. On-chip oscillator circuits vary widely in terms of required specifications such as oscillation frequencies, frequency accuracy and allowable power consumption.
For example, Japanese Patent Laying-Open No. 2006-086997 discloses the following oscillation circuit. That is, an oscillator oscillating at a frequency according to a control signal to generate an output signal, a frequency-to-voltage converter generating a detection signal of a voltage according to a frequency of the output signal from the oscillator, a difference detector generating a difference signal indicating a difference between the detection signal and a reference signal and an integrator integrating the difference signal to generate a control signal are connected in a closed loop.
Moreover, Japanese Patent Laying-Open No. 06-303133 discloses the following oscillation circuit. That is, the oscillation circuit has a self-propelled oscillation unit, and outputs an oscillation signal. The oscillation circuit includes an oscillator having a frequency of an output controlled by a control signal, a first frequency-to-voltage converter receiving the output from the oscillator or a signal obtained by dividing the output from the oscillator, a second frequency-to-voltage converter receiving a reference frequency signal or a signal obtained by dividing the reference frequency signal, and a subtracter receiving an output from the first frequency-to-voltage converter and an output from the second frequency-to-voltage converter and outputting a difference between the output from the first frequency-to-voltage converter and the output from the second frequency-to-voltage converter. Herein, the oscillator receives the output from the subtracter or a signal obtained by amplifying the output from the subtracter. A frequency of the output from the oscillator is controlled by a frequency of the reference frequency signal.
However, each of the oscillation circuit disclosed in Japanese Patent Laying-Open No. 2006-086997 and the oscillation circuit disclosed in Japanese Patent Laying-Open No. 06-303133 integrates a signal having a voltage level which changes at every cycle that an oscillation frequency is converted into a voltage. Consequently, the integrator requires a rapid response and increases in amount of electric current to be consumed.